Synthesis of Highly Enantio-Enriched α-Amino Acids by Carboxylation ofN-(α-Lithioalkyl)oxazolidinones

N-(α-Stannylalkyl)oxazolidinones can be obtained as a mixture of diastereomers in three steps from aldehydes with yields dependent on the R group of R-CHO. They can be transformed by a tin-lithium exchange to N-(α-lithioalkyl)oxazolidinones which equilibrate rapidly to one diastereomer. These compounds give rise, after carboxylation, to the diastereopure N-(α-carboxyalkyl)oxazolidinones. Transformation of the oxazolidinone moiety to a free amino group is accomplished by a Birch-type reduction. Using this method, L-methionine, L-alanine, L-leucine and L-homocysteine were obtained in good yields and ee = 92% to ≥ 95%. The short time required for the whole sequence makes this method ideal for synthesising 1-[11C]amino acids.